Various exhaust after-treatment (AT) devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. One of the exhaust after-treatment devices frequently used in a modern lean burn internal combustion engine, such as a compression-ignition type, is a selective catalytic reduction catalyst (SCR).
The SCR is configured to convert nitrogen oxides (NOx) into diatomic nitrogen (N2) and water (H2O) with the aid of the NO2 generated by another exhaust after-treatment device, typically the diesel oxidation catalyst (DOC). For effective removal of NOx, the SCR conversion process additionally requires a predetermined amount of ammonia (NH3) to be present in the exhaust gas flow.
The SCR conversion process may additionally require a controlled or metered amount of a reductant having a general name of “diesel-exhaust-fluid” (DEF) when the reductant is employed in diesel engines. Such a reductant may be an aqueous solution of urea that includes water and ammonia.